The invention relates to a projection exposure system, in particular for microlithography, for generating an image of an object disposed in an object plane in an image plane by means of a light source emitting projection light, having illuminating optics disposed in the beam path between the light source and the object plane and having projection optics disposed in the beam path between the object plane and the image plane, wherein there is disposed in the vicinity of a field plane of the illumination optics at least one optical element that changes the angular illumination distribution of the projection light in the object plane, wherein the change, imposed by the optical element, in the angular illumination distribution in the object plane is non-rotationally symmetrical with respect to the optical axis.
The term xe2x80x9cangular illumination distributionxe2x80x9d denotes the energy of the illumination light as a function of the beam direction of the illumination light. Examples of light sources having a rotationally symmetrical angular illumination distribution are a spherically radiating light source or a laser in a TEM00 fundamental mode.
To characterize angular illumination distributions that are non-rotationally symmetrical (or rotationally asymmetrical ones), use is made to a first approximation within the scope of the description below of a quantity that is described as ellipticity. The ellipticity of a projection light beam containing rays having different direction is determined by projection of the direction vector beam representing the ray directions of the projection light beam in a plane perpendicular to the optical axis. Said plane in which all direction vectors of the direction vector beam intercept is subdivided for this purpose into four sectors, in this case therefore into quadrants, that are enumerated, for example, from 1 to 4 in the clockwise direction.
In this connection, every direction vector has a length that corresponds to the projection light energy radiated in said direction. To determine the ellipticity, the resulting projections of the direction vectors of the direction vector beam are separately integrated in the four quadrants, and this results in the energy integration values E1 to E4 for the four quadrants.
The ellipticity is then given as the ratio:
(E1+E3)/(E2+E4). 
The deviation of the value of the ellipticity from 1 is a measure of the deviation to a first approximation of the angular illumination distribution from the rotational symmetry. It is clear that division of the projection plane into a greater number of sectors makes possible a correspondingly better description of the symmetry ratio of the angular illumination distribution. Thus, a doubling from 4 to 8 sectors can describe not only a two-fold deviation from rotational symmetry, as is the case for ellipticity, but a 4-fold deviation. For the description below, the ellipticity serves as a non-restrictive example of a non-rotationally symmetrically angular illumination distribution, which may, in practice, also be non-rotationally symmetrical in higher orders (higher multiplicities).
A projection exposure system of the type mentioned at the outset is disclosed in DE 195 20 563 A1. The optical elements that modify the angular illumination distribution in a non-rotationally symmetrical manner serve in this case to shape a projection light beam in such a way that the rectangular entry face of a glass rod used to homogenize the projection light in the illumination optics is illuminated as efficiently as possible.
In the case of projection exposure systems, in particular in the case of those that employ projection light in the deep ultraviolet (DUV), there are a number of mechanisms that result in non-rotationally symmetrical changes in the angular illumination distribution in the illumination of the object, in which connection said changes have, as a rule, to be corrected. Such mechanisms are, for example, a non-rotationally symmetrical light distribution from the light source, optical elements in the illumination optics having non-rotationally symmetrical imaging properties, such as, for example, a non-rotationally symmetrically imaging diffractive optical element, or a glass rod having a non-rotationally symmetrical cross section. Furthermore, depending on the illumination conditions of the illumination or projection optics, the illumination-induced extension or the illumination-induced change in the refractive index of optical components that may modify non-rotationally symmetrical imaging properties of the latter are also relevant. In the case of certain applications of projection exposure systems, for example in the case of reticle structures with preferred directions, a systematically adjustable, non-rotationally symmetrical angular illumination distribution may also be desirable.
The object of the present invention is therefore to develop a projection exposure system of the type mentioned at the outset in such a way that the symmetry of the angular illumination distribution can be modified more flexibly.
This object is achieved according to the invention in that the optical element can be disposed in various angular positions around an axis perpendicular to the field plane.
According to the invention, the degree of rotational asymmetry can be adjusted by the choice of the optical element and by its angular position. This can be utilized, for example, to symmetrize a non-rotationally symmetrical angular illumination distribution produced in other components of the illumination or projection optics. Alternatively, that component of the non-rotationally symmetrical angular illumination distribution that can be adjusted by means of the rotatable optical element can be used for systematically introducing a non-rotationally symmetrical angular illumination distribution for certain illumination purposes.
The optical element may be rotatable around an axis perpendicular to the field plane. Such an optical element can easily be adjusted to various angular positions around an axis perpendicular to the field plane.
Alternatively, to modify the angular illumination distribution of the projection light in the object plane, a plurality of optical elements that can be introduced interchangeably into the beam path of the projection exposure system are provided in an interchange holder. The relative angular position of the individual optical elements may, in this case, already be pre-aligned in the interchange holder so that, when the optical element is introduced into the beam path of the projection exposure system, fine adjustment of the angular position is no longer necessary.
The optical element that can be disposed in various angular positions may be a diffractive optical element. Such optical elements can be designed in such a way that only a little material of the projection light has to pass through them. On the other hand, the imaging action of diffractive optical elements may be varied in wide ranges during their production. The optical element that can be disposed in various angular positions may be a raster element having a two-dimensional raster structure that is made up of a multiplicity of individual structures adjoining one another and having identical area configuration. Such raster elements may function diffractively, but alternatively also refractively in the form, for example, of a lens array or even reflectively. Such optical elements, in which, for example, the non-rotationally symmetrical modification of the angular illumination distribution can be predetermined, can be produced at acceptable cost.
Preferably, the individual structures adjoin one another without gap. In this way, optical elements having high efficiency for the projection light they modify can be constructed.
The individual structures may be designed in such a way that a non-rotationally symmetrical change in the angular illumination distribution they impress is based at least partly on a non-homogeneous modification of the diffraction of the projection light. In this case, the diffraction structures within an individual structure are constructed in such a way that the projection light modified by an individual structure is changed non-rotationally symmetrically in its angular illumination distribution. The shape of the boundary of such individual structures can then be chosen freely within certain limits so that individual structures having particularly simply producible boundary shape can be chosen. Relatively complex deviations in the modification of the angular illumination distribution of the projection light from rotational symmetry can be achieved by means of the configuration of the diffraction structures within an individual structure.
Alternatively, but also additionally, the individual structures can be designed in such a way that the non-rotationally symmetrical change in the angular illumination distribution they impress is based at least partly on a non-rotationally symmetrical boundary of the individual structures. The modification of the angular illumination distribution by the optical element that can be disposed in various angular positions can be predetermined by the area configuration of the individual structures. The more strongly the surface areas of the individual quadrants of the individual structure differ, for example, in the case of an individual structure functioning as a convergent or divergent lens, the greater is the modification of the ellipticity of the angular illumination distribution of an optical element having such individual structures.
An individual structure can have the shape of a hexagon. Such optical elements are known. Since the surface areas of the quadrants of a hexagon differ only relatively little, a fine adjustment of the angular illumination distribution is possible with such individual structures. Alternatively, the individual structures may also have other shapes, for example be designed as hexagons having mutually opposite extended sides, as rectangles or as other structures with which a preferably gapless area structure is possible.
A preferred refinement of the invention has at least two optical elements that are each disposed in the vicinity of a field plane of the illumination optics which change the angular illumination distribution of the projection light passing through and can be disposed, independently of one another, in various angular positions around an axis perpendicular to the field plane. Such an arrangement increases the number of adjustable degrees of freedom for predetermining a certain angular illumination distribution. In particular, the degree of the rotational asymmetry produced by the two optical elements can be adjusted.
In this connection, a change in the symmetry of the angular illumination distribution does not result in a relative position of the optical elements. With an arrangement that permits such a xe2x80x9cneutral positionxe2x80x9d, the symmetry of the angular illumination distribution of the remaining optical components of the projection exposure system can easily be determined for checking purposes.
Preferably, a drive device is provided that is coupled to the optical element that can be disposed in various angular positions. Such a drive device designed, for example, as a stepping motor permits a reproducible adjustment of the position of the optical element.
Additionally, a control device may be provided that operates together with the drive device and that, independently of a predetermined angular illumination distribution of the projection light in the object plane, stimulates the drive device to set a predetermined position of the optical element that can be disposed in various angular positions around an axis perpendicular to the field plane. The adjustment of the angular illumination distribution in this way after an appropriate input has taken place can be automated. In this connection, the input may take place, for example, by automatic reading of information assigned to the object so that the projection exposure method can proceed in an automated manner even if the nature of the object is changed.
In this connection, at least one detector device, operating together with the control device, may be provided for determining the intensity distribution of the projection light in a plane perpendicular to the optical axis. With the aid of such a detector device, the intensity distribution of the projection light can be monitored and conclusions can be drawn therefrom about the effect of the optical element or elements that can be disposed in various angular positions.
The detector device may be a two-dimensional CCD array. CCD arrays are photosensitive and have a high positional resolution.
The detector device may be designed in such a way that it determines the angular illumination distribution of the projection light in the object plane. This makes possible a direct feedback in which the measured angular illumination distribution is compared with a set-point value. This information can then be used to correct the angular illumination distribution with the aid of the optical element or elements that can be disposed in various angular positions. If an optical element is used that can be disposed in various angular positions by means of a drive device, the feedback may take place in a control loop.